Executive Summary
Forensic evidence, particularly fingerprints, has been used for more than a century to aid law enforcement investigations. However, only in the past decade has the use of deoxyribonucleic acid (DNA) testing to include or eliminate suspects and exonerate those convicted in serious crimes become relatively common. DNA evidence is said to be individualizing because of its power to link a person to a criminal incident. And unlike fingerprints, the probability that questioned evidence from a crime scene matches DNA from a known person can be calculated. In past decades, the investigation of serious crimes that led to a conviction typically did not use individualizing forensic biological evidence such as DNA. Thus, it is possible that some individuals convicted in serious person crimes (sexual assault and homicide) would have been eliminated by a forensic analysis more discriminating than what was available at the time. To estimate the rate of such possible wrongful convictions and to identify their predictors, the National Institute of Justice (NIJ) in the U.S. Department of Justice funded retrospective DNA testing of physical evidence in cases where there was a conviction of a sexual assault or homicide and physical evidence was retained.
Two states participated in this research. In Arizona, every eligible prisoner was informed about the program and was given the opportunity to request DNA testing of physical evidence. Testing was performed when physical evidence could be located and if a review of the case deemed that evidence to be probative in the conviction. The Arizona site thus provides a case study that can be used to qualitatively evaluate how often a voluntary program leads to detection of wrongful conviction. The results of the Arizona experiment are described in a separate report.
In Virginia, a cohort of 634 cases of sexual assault and/or homicide dating from 1973 to 1987 was discovered to have retained physical evidence. Since most state legislation that requires evidence storage was enacted in the post-DNA era, it is likely that many states have not preserved physical evidence for cases from the pre-DNA era. Therefore, the evidence in the Virginia cases provides a unique opportunity to determine how often DNA testing can be used to identify wrongful convictions. The results can be generalized (with caveats) because the physical evidence was retained for reasons unrelated to the case outcome, and the cases were assigned to the serologist who retained the evidence in a way that did not introduce bias.
Once cases were found to meet the NIJ eligibility requirements (retained physical evidence, conviction of a sexual assault and/or homicide), the evidence was sent to a private lab for DNA analysis. The goal of this DNA testing was to develop a profile from questioned evidence, generally from the crime scene, and compare it to profiles of known persons developed from the original evidence or stored in a database.1 From these comparisons, a determination can be made whether that evidence is indeterminate,2 inculpatory, or exculpatory.3 The Virginia cases, all of which occurred before DNA evidence testing was readily available, can therefore be used to answer a critical policy question: “What proportion of convicted offenders in serious person crimes with retained forensic evidence could be exonerated if that evidence were DNA tested?”
To answer this question, the Urban Institute (UI) conducted a retrospective study using observational data from the Virginia post-conviction DNA analyses to estimate the rate at which defendants are wrongly convicted and to identify case attributes associated with such wrongful convictions. Toward this end, the Virginia data, which were contained in files maintained by the Virginia Department of Forensic Science (DFS), were made available to UI researchers. All files contained information about the pre- and post-conviction forensic facts of the case, including results of the original forensic testing on the physical evidence, as well as results of the contemporary DNA analysis. Additionally, most files contained information about other basic case attributes, such as the charge and jurisdiction of the crime and demographic information about the convicted offender and other known suspects and victims. These data will serve two purposes. ..continued.. by John Roman, Ph.D., Kelly Walsh, Ph.D., Pamela Lachman, Jennifer Yahner
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